Themselves. The most boring people are completely focussed on themselves. But the absolute worst are the people who show off – who see every conversation as an opportuity to boost their ego and self-esteem.
Politics. I can't bear smug left-wing robots (or lib-bots, as I call them) who mindlessly regurgitate the standard liberal-left view on everything. No, it doesn't make you a cool, edgy outsider. To be counter-cultural nowadays you'd have to be a patriotic conservative!
Cars
Mortgage rates
Work
Football (and sport generally)
Also children. Nothing on earth is so boring as listening to the 'hilarious' thing their five-year-old said.
Interesting things (to me anyway): your favourite paintings, your recent trip to an art gallery, places you've lived/visited abroad, your favourite books, literature in general, history, philosophy, science, etc.